Process for producing diene polymer solutions in vinyl aromatic monomers

ABSTRACT

The anionic polymerization of dienes or copolymers of dienes and vinylaromatic monomers in a vinylaromatic monomer or monomer mixture to give homopolydienes or copolymers or mixed homopolydienes and copolymers is carried out in the presence of a metal alkyl or aryl of an element having a valence of at least two without addition of Lewis bases.

The present invention relates to a process for the anionicpolymerization of dienes or copolymerization of dienes and vinylaromaticmonomers in a vinylaromatic monomer or monomer mixture to givehomopolydienes or copolymers or mixed homopolydienes and copolymers.

The invention further relates to a diene polymer solution, its use forpreparing molding compositions comprising vinylaromatic monomers andalso a continuous process for preparing impact-modified, thermoplasticmolding compositions.

It is generally known that anionic polymerization proceeds completely,ie. to complete conversion, but also very quickly. The conversion ratecan, apart from selection of a very low temperature, only be reduced byselecting a lower concentration of the polymerization initiator, butthis forms only few, very long chain molecules. Owing to theconsiderable evolution of heat and the difficulty of removing the heatfrom a viscous solution, limiting the reaction temperature is not veryeffective.

An excessively high reaction temperature has particularlydisadvantageous consequences, especially in block copolymerization,because thermal termination interferes with the formation of uniformblock copolymers and, if it is intended to follow the polymerizationwith a coupling reaction, the coupling yield would be unfavorably low.

The temperature therefore has to be controlled by appropriate dilutionof the monomers, but this makes the reaction space requiredunnecessarily large, ie. the anionic polymerization can, despite thehigh reaction rate with can be achieved, only be operated with arelatively low space-time yield.

Various continuous and batches processes in solution or suspension areknown for preparing high-impact polystyrene. In these processes, arubber, usually polybutadiene, is dissolved in monomeric styrene whichhas been polymerized to a conversion of about 30% in a preliminaryreaction. The formation of polystyrene and the simultaneous decrease inthe concentration of monomeric styrene leads to a change in the phasecoherence. During this phenomenon known as “phase inversion”, graftingreactions also occur on the polybutadiene and these, together with theintensity of stirring and the viscosity, influence the formation of thedisperse soft phase. In the subsequent main polymerization, thepolystyrene matrix is built up. Such processes carried out in varioustypes of reactor are described, for example, in A. Echte, Handbuch dertechnischen Polymerchemie, VCH Verlagsgesellschaft Weinheim 1993, pages484-489 and U.S. Pat. Nos. 2,727,884 and 3,903,202.

In these processes, the separately prepared rubber has to be comminutedand dissolved in a complicated procedure and the polybutadiene rubbersolution in styrene obtained in this way has to be filtered to removegel particles before the polymerization.

Various attempts have therefore been made to prepare the necessaryrubber solution in styrene directly by anionic polymerization ofbutadiene or butadiene/styrene in nonpolar solvents such as cyclohexaneor ethylbenzene and subsequent addition of styrene (GB 1 013 205, EP-A-0334 715 and U.S. Pat. No. 4,153,647) or by incomplete conversion ofbutadiene in styrene (EP-A 0 059 231). The block rubber thus preparedeither has to be purified by precipitation or else the solvent and othervolatile materials, in particular monomeric butadiene, have to bedistilled off. In addition, owing to the high solution viscosity, onlyrelatively dilute rubber solutions can be handled, which results in ahigh solvent consumption, purification costs and energy consumption.

EP-A 0 304 088 describes a process for the selective polymerization ofconjugated dienes in a mixture of dienes and vinylaromatic compounds.The catalysts used display virtually no polymerization activity towardthe vinylaromatic compound.

U.S. Pat. No. 3,264,374 describes the preparation of polybutadiene instyrene. However, the experiments reported there without any indicationof the scale cannot be controlled on an industrially relevant scalebecause of the abovementioned problems of heat removal. In addition, theviscosities are very high at the relatively low reaction temperatures.

The influence of Lewis acids and Lewis bases on the reaction rate inanionic polymerization has been described in Welch, Journal of theAmerican Chemical Society, Vol 82 (1960), pages 6000-6005. It was foundthat small amounts of Lewis bases such as ethers and amines acceleratethe n-butyllithium-initiated polymerization of styrene, while Lewisacids such as zinc alkyls and aluminum alkyls can reduce thepolymerization rate. Hsieh and Wang too, in Macromolecules, Vol 19(1966), pages 299-304 describe the polymerization-retarding action ofdibutylmagnesium by means of complex formation with the alkyllithiuminitiator or the living polymer chain without influencing thestereochemistry.

U.S. Pat. No. 3,716,495 discloses initiator compositions for thepolymerization of conjugated dienes and vinylaromatics, in which a moreeffective utilization of the alkyllithium as initiator is achieved byaddition of a metal alkyl such as diethylzinc and polar compounds suchas ethers or amines. Owing to the large amounts of solvent required,relatively low temperatures and long reaction times in the range of afew hours, the space-time yields are correspondingly low.

U.S. Pat. No. 3,826,790 discloses a process for the polymerization ofconjugated dienes and, if desired, monovinylaromatic hydrocarbons insolution to give polymers having an increased cis-1,4 content. Theinitiator employed for this purpose contains an alkyllithium compoundand a trihydrocarbylboron compound.

It is an object of the present invention to find a process for theanionic polymerization of dienes and vinylaromatic monomers which, athigh monomer concentration, can be operated particularly economicallyand enables vinylaromatic-monomer solutions of diene polymers which arelow in diene monomers to be prepared for further processing to givemolding compositions. The process should use predominantly monomericstarting materials and make it possible to achieve high space-timeyields. Furthermore, reliable control of the polymerization rate andthus the temperature should be made possible. In addition, the inventionshould provide a continuous process for preparing impact-modifiedmolding compositions having a low residual monomer content.

We have found that this object is achieved by a process for the anionicpolymerization of dienes or copolymerization of dienes and vinylaromaticmonomers in a vinylaromatic monomer or monomer mixture to givehomopolydienes or copolymers or mixed homopolydienes and copolymers,wherein the polymerization is carried out in the presence of a metalalkyl or aryl of an element having a valence of at least two withoutaddition of Lewis bases.

Furthermore, we have found a diene polymer solution, its use forpreparing molding compositions comprising vinylaromatic monomers, inparticular high-impact polystyrene, acrylonitrile-butadiene-styrenepolymers and methyl methacrylate-butadiene-styrene copolymers and also aprocess for the continuous preparation of impact-modified, thermoplasticmolding compositions which comprise a soft phrase comprising a dienepolymer dispersed in a hard vinylaromatic matrix, which comprises

preparing, as described in the introduction, the diene polymer requiredfor the formation of the soft phase in a first reaction zone,

feeding the diene polymer obtained in this way, either directly or afteraddition of a termination or coupling agent, to a second reaction zonein which, if desired with addition of further vinyl monomers in anamount which is sufficient to achieve phase inversion and, if desired,further initiators and/or solvents, anionic or free-radicalpolymerization is carried out until phase inversion occurs and

in a third reaction zone, anionic or free-radical polymerization iscontinued until complete with the amount of vinylaromatic monomerrequired to form the impact-modified thermoplastic molding composition.

The process can be applied to the customary anionically polymerizablediene monomers which meet the usual purity requirements, especially theabsence of polar substances.

Preferred monomers are butadiene, isoprene, 2,3-dimethylbutadiene,1,3-pentadiene, 1,3-hexadiene or piperylene or mixtures thereof.

Suitable vinylaromatic monomers are, for example, styrene,α-methylstyrene, p-methylstyrene, ethylstyrene, tert-butylstyrene,vinyltoluene, vinylnaphthalene or 1,1-diphenylethylene or mixturesthereof. These can be used as comonomers and also as “solvents” or“solvent component” for the process of the present invention. Particularpreference is given to using styrene as “solvent” or “solventcomponent”.

The diene is generally used in amounts of 2-70% by weight, preferably5-35% by weight, particularly preferably 15-25% by weight, based on thesum of all monomers.

For practical reasons, a small amount of a further solvent can be used.Suitable further solvents are the aliphatic, cycloaliphatic or aromatichydrocarbons having from 4 to 12 carbon atoms which are customary foranionic polymerization, for example pentane, hexane, heptane,cyclohexane, methylcyclohexane, isooctane, benzene, alkylbenzenes suchas toluene, xylene and ethylbenzene or decalin or suitable mixtures. Thesolvent should naturally have the high purity typically required forsuch a process. To remove proton-active substances, they can be, forexample, dried over aluminum oxide or molecular sieves and distilledbefore use. The solvent from the process is preferably reused aftercondensation and the purification mentioned. If a further solvent isused, the amount added is generally less than 40% by volume, preferablyless than 20% by volume and very particularly preferably less than 10%by volume, based on the vinylaromatic monomer or monomer mixture.

Initiators used are the monofunctional, bifunctional or multifunctionalalkali metal alkyls or aryls customary for anionic polymerization. Useis advantageously made of organolithium compounds such as ethyllithium,propyllithium, isopropyllithium, n-butyllithium, sec-butyllithium,tert-butyllithium, phenyllithium, diphenylhexyllithium,hexamethylenedilithium, butadienyllithium, isoprenyllithium orpolystyryllithium or the multifunctional organolithium compounds1,4-dilithiobutane, 1,4-dilithio-2-butene or 1,4-dilithiobenzene. Theamount of initiator required is generally in the range from 0.002 to 5mol percent, based on the amount of monomer to be polymerized.

As metal alkyl or aryl of an at least divalent element, use isadvantageously made of compounds of the formula (I)

R_(n)M  (I)

where

M is an element of main group II or III or transition group II of thePeriodic Table,

R is hydrogen, halogen, C₁-C₂₀-alkyl or C₆-C₂₀-aryl, where radicals Rcan be identical or different, and

n is 2 or 3, corresponding to the valence of the element M.

M is preferably one of the elements Be, Mg, Ca, Sr, Ba, B, Al, Ga, In,Tl, Zn, Cd, Hg, particularly preferably magnesium, aluminum, boron orzinc. Possible radicals R are, in particular, hydrogen, halogen andC₁-C₁₂-alkyl, for example ethyl, propyl, n-, i- or t-butyl, octyl ordodecyl, and also C₆-C₁₀-aryl, for example phenyl. Very particularpreference is given to using the commercially available productsdiethylzinc, butylethylmagnesium, dibutylmagnesium, dihexylmagnesium,butyloctylmagnesium, triisobutylaluminum, trihexylaluminum,triethylaluminum, trimethylaluminum, diethylaluminum chloride anddiethylaluminum hydride. Of course, it is also possible to use mixturesof compounds of the formula (I).

Addition of an organometallic compound of an element having a valence ofat least two enables the reaction rate to be reduced sufficiently forthe heat of polymerization to be controlled even at a high monomerconcentration without adverse effects on the polymer properties, sothat, for example, it is possible to carry out an isothermalpolymerization while at the same time achieving a high space-time yield.

The ratio of the initiator to the metal alkyl or aryl according to thepresent invention depends on the desired polymerization temperature andpolymerization rate. The metal alkyl or aryl according to the presentinvention is used, for example, in a molar ratio of from 0.5:1 to 50:1,preferably from 1:1 to 30:1, particularly preferably from 1:1 to 10:1,based on the amount of initiator. Since the molecular weight of thepolymers does not depend only on the molar amount of the initiator, asis usually the case in anionic polymerization, but can also beinfluenced by the type and amount of the metal alkyl or aryl usedaccording to the present invention, preliminary experiments areadvantageously carried out. If, for example, the combination ofbutyllithium as initiator and dibutylmagnesium as component forcontrolling the reaction rate is employed, the molar ratio of Li:Mgselected is advantageously from 0.02:1 to 2:1.

In a preferred embodiment, the alkali metal alkyls used as initiatorsare dissolved together with the metal alkyl or aryl compounds employedaccording to the present invention in a hydrocarbon, for examplen-hexane, n-heptane or cyclohexane, and added in the first reaction zoneto initiate the polymerization. If desired, a solubilizer, for exampletoluene, ethylbenzene, xylene or diphenylethylene, can be added at thusjuncture in order to prevent precipitation of one of the components fromthis initiator solution.

To prevent formation of wall deposits and “popcorn”, gel inhibitors suchas hydrocarbon halides, silicon halides and 1,2-diolefins may, ifdesired, also be added to the polymerization mixture. The amounts useddepend on the compound employed in the particular case. The preferred1,2-butadiene is generally used in amounts of from about 100 to about3000 ppm.

After the build-up of the molecular weight is complete, the “living”ends of the polymer can be reacted with the chain termination orcoupling agents customary for anionic polymerization.

Suitable chain termination agents are proton-active substances or Lewisacids such as water, alcohols, aliphatic and aromatic carboxylic acidsand also inorganic acids such as carbonic acid or boric acid.

For coupling the rubbers, it is possible to use polyfunctional compoundssuch as polyfunctional aldehydes, ketones, esters, anhydrides orepoxides, giving polydienes having a doubled molecular weight orbranched or star-shaped diene polymers.

Both homopolymers and also copolymers or block copolymers are obtainablefrom dienes using the process of the present invention. Preference isgiven to preparing homopolymers, copolymers and block copolymerscomprising butadiene or isoprene. Particular preference is given topolybutadiene and butadiene-styrene block copolymers. The homopolymers,copolymers and block copolymers preferably have molecular weights in therange from 10,000 to 10,000,000 g/mol, particularly preferably from20,000 to 500,000 g/mol and very particularly preferably from 50,000 to300,000 g/mol. The molecular weights can be controlled by the type andamount of initiator, metal alkyls used according to the presentinvention, temperature and conversion. Mixtures of homopolymers andcopolymers can, for example, be obtained by introducing initiator at anumber of different times.

Under said reaction conditions, the diene blocks contain small amountsof copolymerized vinylaromatic compounds which are asymmetricallydistributed over the molecule. The amount of these which is presentincreases with rising conversion (Hsieh et al, Rubber, Chem. Tech. 1970,43, 22). If a homopolydiene containing a very small amount ofcopolymerized vinylaromatic monomers is desired, the polymerization isadvantageously stopped at a conversion of only 35-95%, preferably40-85%, based on the dienes. The unreacted diene monomers are separatedoff, for example by venting, and can thus be returned to the processafter condensation.

In a further embodiment of the process of the present invention, forexample, the diene monomers are dissolved in the vinylaromatic compoundand the polymerization is carried out in the presence of theabovementioned initiators and the polymerization-retarding metal alkylcompound until complete conversion, based on the diene monomers, isreached. As soon as the diene monomers are consumed, which can berecognized (visually or by means of a UV sensor) by the change in colorof the reaction solution from yellow to red, the polymerization reactioncan be stopped by addition of the abovementioned chain termination orcoupling agents.

If the reaction is not stopped at the color change, ie. after the dienemonomers are consumed, but at a later point in time, a block comprisingvinylaromatic monomers which adjoins the diene block is obtained. Thisgives block copolymers or, if further diene monomers are added orcoupling agents are used, multiblock copolymers or star polymers.

However, it is also possible to carry out the reaction to incompleteconversion, particularly in a continuous process, if the remainingmonomers do not interfere in the intended use or subsequent reaction. Ingeneral, however, a conversion of at least 70%, preferably at least 80%,and very particularly preferably complete conversion, is desired if noremoval of unreacted diene monomers is to be carried out.

For the purposes of the present invention, complete conversion means aconversion of more than 96% by weight, based on the diene component. Atthis degree of conversion the content of residual monomers is so lowthat a subsequent free-radical reaction of the reaction mixture does notlead to interfering crosslinking reactions. A conversion of more than100% by weight based on the diene monomers, which means the formation ofa vinylaromatic block adjoining the diene block, can be desirable if onewishes to obtain a diene polymer having a certain compatibility with amatrix different from the diene polymer. This can be important, inparticular, if the reaction mixture is subjected to a subsequent anionicpolymerization in which no grafting reactions with the diene polymeroccur. In this case, the polymerization is continued until theincorporation of vinylaromatic monomers is generally in the range from0.1 to 100% by weight, preferably from 5 to 50% by weight, based on thediene polymer.

A further process variant comprises adding the diene monomers to thereaction mixture only at a later point in time after a block ofvinylaromatic monomers has already been formed. This gives, for example,styrene-butadiene block copolymers. If further initiator is also addedsimultaneously with the diene monomers, it is also possible to obtainrubber solutions in the vinylaromatic compound, comprising mixtures ofhomodienes and block copolymers, which solutions are, in particular,suitable for the subsequent anionic polymerization to give high-impactmolding compositions.

The process of the present invention can be carried out in any pressure-and heat-resistant reactor. Technically, it is unimportant whether thereactors are backmixing or non-backmixing reactors (ie. reactors withstirred tank or plug flow behavior). Suitable reactors are, for example,stirred vessels, loop reactors and also tube reactors or tube-bundlereactors with or without internal fittings. Internal fittings can bestatic or movable fittings.

The reaction can be carried out, for example, at from 20° C. to 150° C.,preferably from 30° C. to 100° C. The reaction temperature can either bekept constant or can be increased or decreased in a controlled manner.To achieve high molecular weights M_(n) and narrow molecular weightdistributions, it is not detrimental if the reaction mixture heats upwithin a short time as a result of the reaction enthalpy liberated.

The process of the present invention can be carried out either as abatch or continuous process. In principle, the components of theinitiator composition, the solvent and the monomers can be mixed withone another in different orders. For example, all starting componentscan be charged initially and solvent and monomers can be addedsubsequently. Alternatively, the components of the initiator system canbe added to the monomer solution either in separate solutions,simultaneously or in succession, or as a mixture prepared in an inertsolvent or solvent system. In the batch method, the monomers can beadded all at once, stepwise or continuously.

Particularly in the continuous procedure, it has been found to beadvantageous to introduce initiator system and monomer solutionsimultaneously or virtually simultaneously, if desired under turbulentmixing conditions, into the reaction vessel. For this purpose, themonomer solution and the initiator solution are mixed, eg. in a mixingnozzle having a small volume under turbulent flow conditions, andsubsequently passed through a tube having a narrow cross section, whichmay be equipped with static mixers (eg. SMX mixers from Sulzer). Theflow rate should be sufficiently high for a relatively uniform residencetime to be observed. The addition of a second monomer can be carried outin a further, downstream mixing nozzle.

For the continuous procedure, it is possible to use continuouslyoperated stirred vessels or loop reactors, or else tube reactors orvarious reactor combinations. In the continuous procedure, preference isgiven to tube reactors, since this enables more uniform products to beobtained because of the residence time spectrum of the reaction mixture.Polymerization in two reaction zones, for example, can also beadvantageous in a continuous process. The first reaction zone serves forthe prepolymerization and is designed as a backmixing unit which isprovided with a heat exchanger. It can, for example, be configured as astirred vessel or as a circulation reactor with static mixers. Ahydraulically filled circulation reactor can be particularlyadvantageous if the reaction mixture has a high viscosity. The desiredconversion generally depends on the viscosity of the reaction mixtureand its handlability. A high value for the conversion is advantageouslyselected so that the residence time to complete polymerization of thereaction mixture in the downstream tube reactor is as short as possibleand the maximum temperature is as low as possible and thus damage ordepolymerization reactions do not occur to any appreciable extent. Inthis first reaction zone, polymerization is advantageously carried outto a conversion of from 30 to 80% by weight, preferably from 40 to 60%by weight.

The diene polymer solution in a vinylaromatic monomer or monomer mixtureobtained by the process of the present invention is low in specks andcan, in principle, be used in all processes in which diene polymersolutions in vinylaromatic compounds are used, which is customarilyachieved by dissolving the diene polymer in the vinylaromatic compoundand/or additional solvents and/or further monomers. Complicatedpurification or filtration is therefore generally not necessary. Thediene polymer solutions can, for example after chain termination butalso without a discrete termination step, if desired after addition offurther monomers, also of vinylaromatic compounds, various ethylenicallyunsaturated compounds, solvents and/or initiators, be subjected directlyto an anionic polymerization or a free-radical polymerization initiatedthermally or by means of free-radical initiators.

The diene polymer solutions are particularly suitable for preparingmolding compositions comprising vinylaromatic monomers, for examplehigh-impact polystyrene (HIPS), acrylonitrile-butadiene-styrene polymers(ABS) and methyl methacrylate-butadiene-styrene copolymers (MBS).

As monomers for forming the hard matrix of the molding compositions, itis possible to add to the diene polymer solution not only theabovementioned vinylaromatic monomers but also further ethylenicallyunsaturated compounds, in particular aliphatic vinyl compounds such asacrylonitrile, acrylic or methacrylic esters, for example the methyl,ethyl, ethylhexyl or cyclohexyl esters, maleic esters, maleic anhydrideor maleimide.

Suitable free-radical initiators are peroxides, for example diacyl,dialkyl or diaryl peroxides, peroxyesters, peroxydicarbonates, peroxyketals, peroxosulfates, hydroperoxides or azo compounds. Preference isgiven to using dibenzoyl peroxide, 1,1-di-tert-butylperoxycyclohexane,dicumyl peroxide, dilauryl peroxide and azobisisobutyronitrile.

Suitable anionic initiators are the alkali metal alkyls mentioned abovefor the diene polymer synthesis.

Auxiliaries which may be added are molecular weight regulators such asdimeric α-methylstyrene, mercaptans such as n-dodecyl mercaptan ortert-dodecyl mercaptan, chain branching agents, stabilizers andlubricants.

The polymerization of the matrix can be carried out in the same pass inbulk or in solution. The polymerization is generally carried out at from50 to 200° C., preferably from 90 to 150° C., in the case offree-radical polymerization, or from 20 to 180° C., preferably from 30to 80° C., in the case of anionic polymerization. The reaction can becarried out isothermally or adiabatically.

The process of the present invention offers the advantage that themolding compositions can be prepared without a costly change of reactionmedium. In addition, no solvents or only small amounts of solvents arerequired, so that their costs and the costs of purification and work-upare largely saved.

In a particular embodiment, the preparation of the diene polymersolution and the polymerization of the matrix are carried out in asingle-pass continuous process. For this purpose, for example, the dienerubber required for formation of the soft phase is polymerized asdescribed above in a first reaction zone and is passed directly or afteraddition of a termination or coupling agent to a second reaction zone.In this second reaction zone, further vinylaromatic or olefinic monomersmay, if desired, be added in an amount which is sufficient to achievephase inversion and, if desired, further anionic or free-radicalinitiators and, if desired, solvents may be added and polymerization iscarried out until phase inversion occurs. In a third reaction zone,anionic or free-radical polymerization is carried out to completion withas much vinylaromatic or olefinic monomer as is required to form theimpact-modified thermoplastic molding composition.

Owing to the relatively low residual monomer content, the rubber isadvantageously polymerized in a tube reactor or a reactor arrangementconcluding with a tube reactor and the rubber solution is continuouslytransferred to a polymerization apparatus of the type used, for example,for preparing high-impact polystyrene and described in A. Echte,Handbuch der technischen Polymerchemie, VCH Verlagsgesellschaft Weinheim1993, pages 484-489.

The molding compositions obtained can be freed of solvents and residualmonomers in a customary manner by means of degassers or degassingextruders at atmospheric pressure or reduced pressure and at from 190 to320° C.

If the matrix of the rubber-modified molding composition is also builtup by anionic polymerization, it may be advantageous to crosslink therubber particles by means of appropriate temperatures and/or by additionof peroxides, in particular those having a high decompositiontemperature, for example dicumyl peroxide.

EXAMPLES Example 1

920 g of styrene, 80 g of butadiene, 50 g of ethylbenzene and a premixedcatalyst solution comprising 0.1 ml of a 1 molar s-butyllithium solutionin cyclohexane and 0.6 ml of a 1 molar dibutylmagnesium solution inn-hexane were added simultaneously over a period of one hour to a 2 lstirred reactor and polymerized at 70° C. The reaction mixture was heldat 70° C. until all the butadiene had been consumed, which could berecognized by the change in color of the reaction solution from yellowto red. At the color change point, the reaction was stopped using 0.1 mlof a 1:1 mixture of methanol and ethanol. This gave a viscous 7.7%strength solution of polybutadiene having a molecular weight ofM_(n)=114,000 in styrene/ethylbenzene.

Examples 2-4

Example 1 was repeated using the parameters shown in Table 1.

Example 5

920 g of styrene, 80 g of butadiene, and a premixed catalyst solutioncomprising 0.1 ml of a 1 molar s-butyllithium solution in cyclohexaneand 0.6 ml of a 1 molar dibutylmagnesium solution in n-hexane were addedsimultaneously over a period of one hour to a 2 l stirred reactor andpolymerized at 70° C. The reaction mixture was held at 70° C. beyond thecolor change from yellow to red. At a solids content of 9.6% by weight,the reaction was stopped using 1 ml of a 1:1 mixture of methanol andethanol. This gave a viscous solution of a butadiene-styrene blockcopolymer having a residual butadiene content of 1720 ppm and amolecular weight of M_(n)=137,000.

Examples 6-8

Example 5 was repeated using the parameters shown in Table 1.

Example 9

5100 g of styrene and 900 g of butadiene were placed in a 10 l stirredreactor and heated to 70° C. At this temperature, a premixed catalystsolution comprising 4.3 ml of a 1.4 molar s-butyllithium solution incyclohexane and 9 ml of a 1 molar dibutylmagnesium solution in n-hexanewas added. After 4.5 hours at this temperature, the reaction was stoppedusing 2 ml of ethanol. This gave a clear, viscous and speck-freesolution having a solids content of 18.1% by weight. The polymerobtained had a molecular weight of 111,000 g/mol. The width of thedistribution M_(w)/M_(n) was 1.20. ¹H-NMR analysis (CDCl₃; 300 MHz)showed a 1,2-vinyl content in the rubber of 10.6 mol%, based on thebutadiene content, and a styrene content of 25% by weight, based on thetotal polymer. IR analysis indicated a cis/trans ratio of the butadienecomponent of 1/1.55. The glass transition temperature of the polymer was−71° C.

Example 10

4250 g of styrene and 750 g of butadiene were placed in a 10 l stirredreactor and heated to 80° C. At this temperature, a premixed catalystsolution comprising 3 ml of a 1.4 molar s-butyllithium solution incyclohexane and 5.1 ml of a 1 molar dibutylmagnesium solution inn-hexane was added. After 3 hours at this temperature, the reaction wasstopped using 2 ml of ethanol. This gave a clear, viscous and speck-freesolution having a solids content of 21.4% by weight. The polymerobtained had a molecular weight of 123,000 g/mol. The width of thedistribution M_(w)/M_(n) was 1.66. ¹H-NMR analysis (CDCl₃; 300 MHz)showed a 1,2-vinyl content in the rubber of 11.2 mol%, based on thebutadiene content, and a styrene content of 44% by weight, based on thetotal polymer. IR analysis indicated a cis/trans ratio of the butadienecomponent of 1/1.49. DSC analysis gave two glass transition temperaturesfor the polymer, −67° C. and 102°0 C.

Comparative Example C1

In a 2 l stirred reactor, 900 g of styrene, 100 g of butadiene and acatalyst solution comprising 0.5 ml of a 1 molar s-butyllithium solutionin cyclohexane were simultaneously polymerized over a period of one hourat 50° C. The reaction could be controlled only up to a conversion of60% by weight, based on the butadiene. The temperature then rose sharplywithin a few minutes and only a highly crosslinked rubber could beisolated.

Comparative Example C2

In a 2 l stirred reactor, 920 g of styrene, 80 g of butadiene and acatalyst solution comprising 0.7 ml of a 1 molar s-butyllithium solutionin cyclohexane were simultaneously polymerized over a period of one hourat 30° C. The reaction could be controlled only up to a conversion of54% by weight, based on the butadiene. The temperature then rose sharplywithin a few minutes and only a highly crosslinked rubber could beisolated.

TABLE 1 Preparation of butadiene polymers in styrene Example 1 2 3 4 5 67 8 C1 C2 Styrene [g] 920 920 800 1000 920 867 920 920 900 920 Butadiene[g] 80 100 200 100 80 133 80 80 100 80 Ethylbenzene [g] 50 500 s-BuLi[ml, 0.1 0.1 0.2 0.2 0.1 0.13 0.1 0.1 0.5 0.7 1M solution] (Bu)₂Mg [ml,0.6 0.4 0.8 0.3 0.6 0.53 0.6 0.6 1M solution] Solids 7.7 9.8 20 6.3 9.616 9.8 11.6 content [%] Temperature [° C.] 70 60 60 50 70 70 70 70 50 30Residual 1720 1720 1680 1380 monomers [ppm] Mn [g/mol] *1000 114 200 200200 137 240 140 165 200 114

Examples 11-14

Continuous Preparation of High-Impact Polystyrene by Free-radicalPolymerization of Styrene

For the example carried out continuously, use was made of an arrangementcomprising a 2 l stirred reactor (R1) for preparing the rubber solutionand a downstream tank (R2)-tank(R3)-column(T1)-column(T2) arrangementwith reactor volumes of 3, 5, 10 and 10 l for phase inversion andpreparation of the impact-modified molding composition. The parts of thereactor arrangement were connected to one another by means of a gearpump.

The running-up phase was carried out as described in Example 1 until thecolor change point was reached. The connection to thetank-tank-column-column cascade was then established and thepolybutadiene solution was continuously taken off, admixed with amixture of methanol and ethanol and fed to the tank-tank-column-columncascade. In the equilibrium state, 920 g/h of styrene, 80 g/h ofbutadiene, 50 g/h of ethylbenzene and 1 ml/h of a premixed catalystsolution comprising 0.1 ml of a 1 molar s-butyllithium solution incyclohexane and 0.6 ml of a 1 molar dibutylmagnesium solution inn-hexane were introduced into the 2 l stirred reactor R1 and polymerizedat 70° C. In the first tank (R2), held at 115° C., of thetank-tank-column-column arrangement, styrene was grafted onto thepolybutadiene at a stirrer speed of 70 rpm while metering in 84 mg/h oftert-butyl per-2-ethylhexanoate (TBPEH). Phase inversion occurred in thesecond tank (R3) held at 120° C. at 120 rpm. The polymerization in thefirst column reactor (T1) occurred at 135° C. and a stirrer speed of 70rpm, in the second column reactor (T2) at 145° C. and 40 rpm. The solidscontents were 13.4% by weight in the first tank, 26.1% by weight in thesecond tank and 83.7% at the outlet of the second column.

TABLE 2 Continuous preparation of high-impact polystyrene byfree-radical polymerization of styrene Example 11 12 13 14 Rubberprepared as in  1  2  3  4 Example Reactor R2 21 21 21 51 TBPEH [mg/h]84 80 70 85 Temperatures [° C.] 115/120/ 116/125/ 116/125/ 115/121/R2/R3/T1/T2 135/145 135/145 135/145 135/145 Stirrer speed [rpm] 70/120/70/120/ 70/120/ 70/120/ R2/R3/T1/T2 70/40 70/40 70/40 70/40 Solidscontent [% by 13.4/26.1/ 16.3/29.1/ 26.5/36.1/ 12.2/23.1/ weight]R2/R3/T2 83.7 85.8 86.4 83.4

Examples 15-18

Continuous Preparation of High-Impact Polystyrene by AnionicPolymerization

For the example carried out continuously by anionic polymerization, usewas made of an arrangement comprising a 2 l stirred reactor (R1) forpreparing the rubber solution, a downstream 2 l stirred reactor (R2) forphase inversion and a tube reactor having a length of 2000 mm and aninternal diameter of 10 mm (volume=0.157 1 ). The individual reactorswere connected to one another via gear pumps.

The running-up phase was carried out as described in Example 5 until asolids content of 9.6% by weight was reached. The polybutadiene solutionwas then continuously taken off, admixed with a mixture of methanol andethanol and fed to the second stirred reactor R2. In the equilibriumstate, 920 g/h of styrene, 80 g/h of butadiene and 1 ml/h of a premixedcatalyst solution comprising 0.1 ml of a 1 molar s-butyllithium solutionin cyclohexane and 0.6 ml of a 1 molar dibutylmagnesium solution inn-hexane were introduced into the 2 l stirred reactor R1 and polymerizedat 70° C. In the second stirred reactor R2 which was held at 70° C.,3.86 mmol/h of dibutylmagnesium (DBM) and 0.52 mmol/h ofsec-butyllithium were metered into the block copolymer solution instyrene at a stirrer speed of 70 rpm. The solids content here was 35% byweight. In the downstream tube reactor, the reaction mixture completedits polymerization at 200° C. and a residence time of 10 minutes to asolids content of 97%. The reaction product was degassed, extruded andgranulated.

TABLE 3 Continuous preparation of high-impact polystyrene by anionicpolymerization Example 15 16 17 18 Rubber prepared as in 5 6 7 8 ExampleReactor R2 21 21 71 71 Ethylbenzene [g/h] 5000 2000 s-BuLi [ml, 1Msolution] 0.52 0.36 4.38 5.35 (Bu)₂Mg [ml, 1M solution] 3.86 3.2 Stirrerspeed [rpm] in R2 70 70 70 70 Temperature [° C.] in R2 70 40 40 40Solids content [% by weight] 35 35 5 12 in R2 Tube reactor R3 (Length/2000/10 2000/10 3000/15 3000/15 internal diameter [mm]) Temperature [°C.] in R3 200 200 150 150 Residence time [min] in R3 10 10 8 12 Solidscontent [% by weight] 97 98 15.5 31.5 in R3 Polystyrene M_(n) [g/mol]*1000 200 250 200 158

We claim:
 1. A process for the anionic polymerization of dienes orcopolymerization of dienes and vinylaromatic monomers in a vinylaromaticmonomer or monomer mixture as solvent to give homopolydienes orcopolymers or mixed homopolydienes and copolymers, wherein thepolymerization is carried out in the presence of an initiatorcomposition consisting of an alkali metal alkyl or aryl as initiator anda metal alkyl or aryl of an element having a valence of at least twoselected from the group consisting of magnesium, aluminum, boron andzinc and without addition of Lewis bases and using less than 40% byvolume of a further solvent, based on the vinylaromatic monomer ormonomer mixture.
 2. A process as claimed in claim 1, wherein the metalalkyl or aryl used is a compound of the formula (I) R_(n)M  (I) where Mis magnesium, aluminum, boron or zinc, R are, independently of oneanother, hydrogen, halogen, C₁-C₂₀-alkyl or C₆-C₂₀-aryl and n is 2 or 3,corresponding to the valence of the element M with the proviso, that atleast one radical R is C₁-C₂₀-alkyl or C₆-C₂₀-aryl.
 3. A process asclaimed in claim 2, wherein the metal alkyl used is diethylzinc,butylethylmagnesium, dibutylmagnesium, dihexylmagnesium,butyloctylmagnesium, triisobutylaluminum, trihexylaluminum,triethylaluminum, trimethylaluminum, diethylaluminum chloride ordiethylaluminum hydride.
 4. A process as claimed in claim 1, wherein themetal alkyl or aryl is used in a molar ratio of from 0.5/1 to 50/1,based on the amount of initiator.
 5. A process as claimed in claim 1,wherein the further solvent used is an aliphatic, cycloaliphatic oraromatic hydrocarbon or a mixture thereof.
 6. A process as claimed inclaim 1, wherein the diene used is butadiene or isoprene.
 7. A processas claimed in claim 1, wherein the vinylaromatic monomer used isstyrene, α-methylstyrene, p-methylstyrene, 1,1-diphenylethylene or amixture thereof.
 8. A process as claimed in claim 1, wherein thepolymerization is stopped using a chain termination agent or couplingagent.
 9. A process for the continuous preparation of impact-modified,thermoplastic molding compositions which comprise a soft phasecomprising a diene polymer dispersed in a hard vinylaromatic matrix,which comprises preparing, as claimed in claim 1 the diene polymerrequired for the formation of the soft phase in a first reaction zone,feeding the diene polymer obtained in this way, either directly or afteraddition of a termination or coupling agent, to a second reaction zonein which, optionally with addition of further vinyl monomers in anamount which is sufficient to achieve phase inversion and, optionally,further initiators and/or solvents, anionic or free-radicalpolymerization is carried out until phase inversion occurs and in athird reaction zone, anionic or free-radical polymerization is continueduntil complete with the amount of vinylaromatic monomer required to formthe impact-modified thermoplastic molding composition.
 10. A process asclaimed in claim 1, wherein styrene, acrylonitrile or methylmethacrylateis added to the obtained solution of diene polymer and the mixture issubjected to anionic polymerization or free-radical polymerizationinitiated thermally or by means of free-radical initiators.